Earlier, the space between the discharge tube and the envelope of discharge lamps was filled with inert gas, as shown for instance in the European patent application No. 165,587. The role of the inert gas was to eliminate oxidation of the metal elements in the discharge tube. A further advantage of applying inert gas was that it increased the breakdown voltage in the envelope and due to this no breakdown occured between metal elements of the discharge tube. On the other hand, however, the heat loss due to the convenction flow of the inert gas decreased the efficiency of the whole lamp.
That was the reason that later vacuum has been produced between the envelope and the discharge tube of discharge lamps and the remaining gases were bound by a getter film in the vacuum space after fusing the end of the exhaust tube of the lamp. In this way, the pressure in the vacuum space could be held below 10.sup.5 mbar (see Debreczeni et al: Fenyforrasok, Budapest, 1985, page 153). This pressure could be held during the entire thousand to over ten thousand hours service life of the lamp.
The getter in the envelope has to bind the gases remaining after pumping out the air as well as gases released and escaped from the discharge tube during operation.
During operation of the discharge lamps mainly hydrogen and oxygen are released and therefore it is reasonable to apply getter material which is capable of binding both gases. A barium getter film applied in electronics is highly active, binds the gases quickly and keeps them permanently bound in rather big quantities. In the case of high-pressure discharge lamps, however, the barium-film is warmed by the heat radiated by the discharge tube and, in this way, the hydrogen binding activity is not sufficiently effective (SAES Getter Catalog, 1982, page 28). Therefore, combined (evaporated and metal) getters are generally applied, which are able to bind the remaining gases as well as those ones which release during operation.
According to the U.S. Pat. No. 3,626,229, zirconium, titanium or zirconium-titanium alloys are used for binding hydrogen and maintaining a pressure of below 10.sup.-7 atmosphere. According to U.S. Pat. No. 3,519,864 BaO.sub.2 getter is used for binding hydrogen. The most often applied getter material for maintaining the required pressure is however, a barium-film evaporated to the inner surface of the neck part of the envelope. The pure barium itself is generally produced by heating a BaAl.sub.4 intermetallic alloy into the range of 900.degree.-1000.degree. C. within the envelope. The releasing barium can be evaporated at the above temperature range quickly and without remaining by e.g. induction heating.
German published patent application No. 3,346,130 discloses a discharge lamp wherein such a getter film is produced in the vacuum space between the envelope and discharge tube, at the lower part of the inner surface of the envelope.
The value of the pressure in the envelope of the high-pressure discharge lamps has a considerable influence on the concentration of the contaminating materials in the discharge space as the electric strength of the envelope and in this way, to the ignition characteristics and service life of the lamp, because the gases in the vacuum space penetrate into the discharge tube by diffusion and so they contaminate the discharge space as well. On the other hand, the metal parts in the envelope can easily bind oxygen and, accordingly, a quick oxidation occurs if oxygen is present in vacuum space. If hydrogen can diffuse into the discharge space, it would influence the ignition characteristics and other operation parameters of the lamp.